This application is the U.S. national phase of international application PCT/JP00/08293 filed Nov. 24, 2000, which designated the U.S.
The present invention relates to a spread modulating method and an apparatus using the method in a spread spectrum communications system and a CDMA (Code Division Multiple Access) system, in particular relating to a CDMA spread modulating method and an apparatus using the method for implementing complex QPSK (Quadrature Phase Shift Keying) spread modulation.
Spread spectrum communication and CDMA (Code Division Multiple Access) systems using spread spectrum communication technologies are characterized by having strong resistance to multipath fading, capability of enhancing the data rate, excellent communication quality, high efficiency in frequency usage and the like, so that they are attracting the attention as the next-generation mobile communications system and multimedia mobile communications.
In the spread spectrum communication, the signal to be transmitted is spread into a signal having a bandwidth much wider than that of the original signal on the transmission side and is transmitted. On the reception side, the spectrum-spread signal is reverted back into the signal having the original signal bandwidth. The above features are obtained by this feature.
FIG. 7 is a block diagram showing a transmitter in a conventional spectrum spread communications system. Information 100 to be transmitted is processed through a primary modulator 101 into a data signal D(t) such as a data signal which has been modulated by BPSK (Binary Phase Shift Keying), QPSK(Quadrature Phase Shift Keying) or the like. The data signal D(t) is secondarily modulated by a secondary modulator 102 based on a spread spectrum code C(t) generated from a spreading code generator 103. AnM-sequence, Goldcode, Hadamard code and other codes can be used as the spreading code C(t). The CDMA system makes distinctions between users, cells, data channels, etc., based on the spreading code C(t) generated from spreading code generator 103. Thereafter, at a multiplier 104, the secondary modulated waveform is multiplied by the carrier wave generated from radio carrier wave generator 105 so that it is transformed into a radio frequency wave. The thus transformed carrier wave (baseband transmission signal) is amplified by an amplifier 106 and sent out from antenna 107.
Similarly to the primary modulation there are some techniques such as BPSK, QPSK as the technique for secondary modulation(spread modulation). FIG. 8 is a block diagram showing one example of a conventional secondary modulator. In this secondary modulator, as shown in FIG. 8, data Di and data Dq which are independent from each other on the in-phase channel (ICH) and quadrature channel (QCH) are operated by multipliers 110 and 111 using independent spreading codes Ci and Cq. By this operation, Dixc2x7Ci and Dqxc2x7Cq are obtained as spread signals 112 and 113, respectively. This technique is called a dual-channel QPSK method, which is effective in transmitting independent data streams in parallel. The spread modulation is described in detail in the following literature:
Literature 1: pp. 471-478 in xe2x80x98Spread spectrum communications systemxe2x80x99 written by Mitsuo Yokoyama, published from Kagaku Gijutsu Shuppan-sha.
Next, a complex QPSK spread modulating technique which is more complicated will be described. FIG. 9 is a block diagram showing another example of a secondary modulator for implementing the complex QPSK spread modulation. Here, complex data (Di, Dq) is complex-spread in a complex QPSK processor 121 by complex spreading codes (Si. Sq) so as to produce ICH spread signal Ai and QCH spread signal Aq. This complex QPSK modulation is represented by the following equation (1):
(Di+jDq)xc2x7(Si+jSq)=(Dixc2x7Sixe2x88x92Dqxc2x7Sq)+j(Dixc2x7Sq+Dqxc2x7Si) =Ai+jAqxe2x80x83xe2x80x83(1)
where j is an imaginary unit.
In order to produce the terms on the right side of equation (1), a complex QPSK processor 121 implements the operation between complex data (Di, Dq) and complex spreading codes (Si, Sq) by multipliers 122, 123, 124 and 125. As a result, (Dixc2x7Si), (Dqxc2x7Sq), (Dixc2x7Sq) and (Dqxc2x7Si) in equation (1) are obtained. Then, the results are summed (subtracted) in adders 126 and 127, taking into account the signs in equation (1).
The W-CDMA (Wideband-CDMA) as a next-generation mobile communications scheme implements spread modulation using two kinds of spreading codes. Specifically, a long code having a markedly long symbol period and short code having a short symbol period are used in combination so as to implement spreading and scrambling. The roles of spread demodulation and spreading codes in the W-CDMA are described in detail in the following literatures:
Literature 2: xe2x80x98Next Generation Mobile Radio Access for Multimedia transmission: W-CDMAxe2x80x99 Sawahashi and Adachi, Technical Report of IEICE, SST-98-41, 1998-12;
Literature 3: xe2x80x98Mobile Radio Access Based on Wideband Coherent DS-CDMAxe2x80x99, Ohno, Sawahashi, Doi, Higashi, NTT DoCoMo Technical Journal, Vol.4No3.
Next, a spread modulating method using two kinds of spreading codes, or of the combination of the double-spreading using (Ci, Cq) in FIG. 8 and the complex QPSK modulation using (Si, Sq) in FIG. 9 will be described. Specifically, data signals (Di, Dq) are subjected first to the double-spreading using the spreading codes (Ci, Cq), and then subjected to the complex QPSK modulation using the spreading codes (Si,Sq). This complex QPSK modulation is represented by equation (2).
(Dixc2x7C+JDqxc2x7Cq)xc2x7(Si+JSq)=(Dixc2x7Cixc2x7Sixe2x88x92Dqxc2x7Cqxc2x7Sq)+j (Dixc2x7Cixc2x7Sq+Dqxc2x7Cqxc2x7Si)=Ai+jAqxe2x80x83xe2x80x83(2)
FIG. 10 is a block diagram showing another example of a secondary modulator for implementing this complex QPSK spread modulating method. In the secondary modulator for implementing this complex QPSK modulation shown in FIG. 10, the data signals (Di, Dq) and spreading codes (Ci, Cq) are double-spread through multipliers 110 and 111. In a complex QPSK processor 121, the signals 112 and 113 having undergone double-spreading are subjected to the complex QPSK spread modulation with the other spreading codes (Si, Sq) and the result is supplied to adder/subtractors 126 and 127 for addition (subtraction).
That is, complex QPSK processor 121, in order to produce the right side terms in equation (2), implements operations between complex data (Dixc2x7Ci, Dqxc2x7Cq) and complex spreading codes (Si, Sq) using multipliers 122, 123, 124 and 125. From these operations, the terms (Dixc2x7Cixc2x7Si), (Dqxc2x7Cqxc2x7Sq), (Dixc2x7Cixc2x7Sq) and (Dqxc2x7Cqxc2x7Si) in equation (2) are determined.
Here, when the spreading rate (chip rate) of the spreading codes (Ci, Cq) is equal to that of the other spreading codes (Si, Sq), the spreading codes (Si, Sq) provide a scrambling function, so that the spreading codes (Si, Sq) are also called scramble codes.
The data signal (Di, Dq) in FIG. 10 are independent from each other as already mentioned. For example. Di may be allotted for information data to be transmitted and Dq may be allotted for a control signal. In some cases, the information data Di and control data Dq may be adjusted as to their amplitude ratio by a gain factor G, dependent on their signal importance. FIG. 11 is a block diagram showing a secondary modulator in which control signal Dq is adjusted by a gain factor G.
This secondary modulator, as shown in FIG. 11, the quadrature channel data signal Dq is weighted by a multiplier 131 based on the signal of gain factor G generated from a gain factor controller 136. The data signal weighted with the gain factor G, or the data signal (Di, Gxc2x7Dq) and the spreading codes (Ci, Cq) are double-spread by multipliers 110 and 111, in the same manner as that shown in FIG. 10. Then, the resultant signals are subjected to complex QPSK spread modulation with the other spreading codes (Si, Sq) by means of QPSK processor 121 and adders 126 and 127.
The signals Ai and Ag having undergone complex QPSK modulation are processed through LPFs (low-pass filters) 132 and 133(or root Nyquist filters for suppressing the power leakage to adjacent channels) for limiting the bandwidth of the CDMA transmission signal and are converted into analog signals (Ri. Rq), by means of DACs (digital-to-analog converters) 134 and 135. Thereafter, the analog baseband signal (Ri, Rq) are converted into a radio frequency wave, which is then amplified and sent out as a CDMA signal from the antenna.
In the CDMA complex QPSK spread modulating circuit (secondary modulator) shown in FIG. 11, fine adjustment of the gain factor G for assigning a weight to the digital data needs to allot an increased number of bits for the gain factor G. When the gain factor G is includes, the ICH spreading signal Ai and QCH spreading signal Aq in FIG. 11 are represented as follows:
(Dixc2x7Ci+jGxc2x7Dqxc2x7Cq)xc2x7(Si+jSq)=(Dixc2x7Cixc2x7Sixe2x88x92Gxc2x7Dqxc2x7Cqxc2x7Sq)+j (Dixc2x7Cixc2x7Sq+Gxc2x7Dqxc2x7Cqxc2x7Si)=Ai+jAqxe2x80x83xe2x80x83(3)
In equation (3), the terms including G are of multi-bit operations. Therefore, in order to execute the operation of equation (3), the complex QPSK processor 121 in FIG. 11 should implement multi-bit operations. As a result, spreading signals Ai and Aq also have multi-bit values. Accordingly, LPFs (or root Nyquist filters) 132 and 133 should be of a multi-bit weighting digital filter configuration having multi-bit input and multi-bit output. LPFs (or root Nyquist filters) 132 and 133 are strictly limited in the CDMA system in order to suppress the power leakage to adjacent channels. The details are given in the following literature 4. Literature 4: NTT DoCoMo Technical Journal Vol.6No.3 xe2x80x98Special Issue (1) on W-CDMA System Experiment (1): Mobile Station Overviewxe2x80x99 by Higashi, Tagagi, Yunoki and Takami.
The strict characteristics of LPFs (or root Nyquist filters) 132 and 133 can be achieved by digital filters having many taps with multi-bit weighting. In this way, LPFs (or root Nyquist filters) 132 and 133 need multi-bit input, multi-bit output, multi-bit weighting and many taps, so that there is a problem that the gate scale and its power consumption increase.
The object of the present invention is to provide a CDMA modulating method and an apparatus thereof for implementing CDMA complex QPSK spread modulation, which can be operated on a reduced gate scale with a low power consumption.
The present invention has the following configurations in order to solve the above problem.
The first invention is a CDMA modulating method for processing in-phase and quadrature channel signals to be transmitted by weighting at least one of the signals by a gain factor, passing the signals obtained by complex QPSK spread modulation based on complex spreading codes through low-pass filtration, subjecting the signals to digital-to-analog conversion so as to produce baseband transmission signals, and is characterized in that the weighting with the gain factor is applied either after the low-pass filtration or the digital-to-analog conversion.
The second invention is a CDMA modulating apparatus comprising: at least one complex spreading code generator; a complex QPSK processor for implementing complex QPSK spread modulation of in-phase and quadrature channel signals to be transmitted, based on complex spreading codes generated from the complex spreading code generator; low-pass filters connected to the signal outputs from the complex QPSK processor; digital-to-analog converters for implementing digital-to-analog conversion of the signals having passed through the low-pass filters; gain factor multipliers for assigning a weight of a gain factor to the terms corresponding to the in-phase channel signal or quadrature channel signal to be transmitted; and adders for summing the weighted signals to produce complex QPSK spread modulated signals, and is characterized in that the gain factor multipliers are arranged downstream of the low-pass filters or digital-to-analog converters.
The third invention is characterized in that the low-pass filters are of a root Nyquist characteristic digital filter configuration having 25 to 40 taps with 5 to 8 bits for weighting coefficient quantization.
The fourth invention is characterized in that when the gain factor is assigned to the digital-to-analog converted, transmission signals, the gain factor is digitally controlled so as to assign the weight on the signals through the digital-to-analog converters.
The fifth invention is characterized in that when respective weights are assigned to the in-phase channel and quadrature channel signals to be transmitted, the gain factor to one of the channel signals is used to normalize the gain factor to the other channel signal, so that gain control is performed on one channel signal only.
The sixth invention is a CDMA modulating apparatus comprising: at least one complex spreading code generator; a gain factor multiplier for assigning a weight of a gain factor to the terms corresponding to the in-phase channel signal or quadrature channel signal to be transmitted; a complex QPSK modulator for implementing complex QPSK spread modulation of the channel signal having passed through the gain factor multiplier and the other channel signal, based on complex spreading codes generated from the complex spreading code generator; adders for summing the outputs from the complex QPSK processor to produce complex QPSK spread modulated signals; low-pass filters receiving the produced complex QPSK spread modulated signals; and digital-to-analog converters for implementing digital-to-analog conversion of the signals having passed through the low-pass filters, and is characterized in that the gain factor is made up of a four bit or greater bit signal
The seventh invention is characterized in that the low-pass filters are of a root Nyquist characteristic digital filter configuration having 25 to 40 taps with 5 to 8 bits for weighting coefficient quantization and the number of bits of the digital-to-analog converters is set at 8 to 10.
The CDMA complex QPSK spread modulating apparatus of the present invention implements spreading and scrambling using two kinds of spreading codes. The QCH information data is subjected to dual-channel QPSK spread modulation with a spreading code and then subjected to complex QPSK operation by another spreading code (scramble code). The operational outputs are input to one-bit input root Nyquist filters and then converted by DACs into analog values. The gain factor is analogically operated with the DAC output signals. The DAC output signals are summed appropriately to produce the aimed ICH and QCH transmission baseband signals.
Thus, the gain factor operation, the complex QPSK operating method and the operating techniques and arrangement of the root Nyquist filters and DACs are optimized, whereby CDMA complex QPSK spreading using two kinds of spreading codes based on equation (3) is made possible to be implemented using, the least possible, one-bit signals as the input signals to the complicated root Nyquist filters (digital filters), instead of using multi-bit signals. As a result, it becomes possible to reduce the number of gates required and reduce the power consumption.